The Cyrus IMAP server differs from other IMAP server implementations in that it
is generally intended to be run on sealed servers, where normal users are not
permitted to log in. The mailbox database is stored in parts of the filesystem
that are private to the Cyrus IMAP system. All user access to mail is through
the IMAP, POP3, or KPOP protocols.

The private mailbox database design gives the server large advantages in
efficiency, scalability, and administratability. Multiple concurrent read/write
connections to the same mailbox are permitted. The server supports access
control lists on mailboxes and storage quotas on mailbox hierarchies.

WARNING: mmap(2) is not fully POSIX on OpenBSD so cyrus needs to map_refresh on
every lock which will incure a performance penalty under high load. See this
bugreport for details:
    http://bugzilla.cyrusimap.org/show_bug.cgi?id=3370
